Internal motions in proteins examined by 13C NMR rotating frame spin-lattice relaxation: Chymotrypsinogen

Abstract

The experimental observable for the NMR relaxation technique involving rotating frame spin-lattice relaxation in the presence of an off-resonance rf field consists of the ratio of the intensity of a resonance peak in the presence of an rf field applied off-resonance to the intensity in the absence of the applied field. The intensity ratio ( R) is related to a relaxation rate ( 1 T 1ϱ off ) produced by the off-resonance irradiation. These relaxation parameters are shown to complement the usual spin-lattice relaxation rate ( 1 T 1 ), the linewidth ( W 1 2 ), and the nuclear Overhauser effect (NOE) in studies of protein mobility. The relaxation parameters are calculated and compared for two motional models, one simply involving random isotropic rotational tumbling and the other imposing internal rotation on the isotropic tumbling. Use of all the 13C NMR relaxation parameters to study protein mobility is illustrated for α-chymotrypsinogen A. A satisfactory fit of theoretical predictions to all the experimental relaxation data for the threonine Cβ resonance is obtained only when the theoretical relaxation parameters are calculated for an overall rotational reorientation time of 35 nsec and a correlation time for the internal motion of 20 nsec.